The invention relates in general to hybrid propulsion systems, and in particular to systems and methods for operating a series hybrid electric propulsion system with an auxiliary power unit.
Some vehicles use electric traction motors to propel the vehicle. Typically, the electric traction motors are connected to a link, such as a bus, that provides the motors with power. One or more on-board alternators may be used to provide the power to the link. In certain operating conditions, such as when the vehicle is decelerating or is maintaining speed on a downhill grade, the back-emf produced by the electric motors is greater than the voltage provided by the engine-driven alternator. Under such conditions, the electric traction motors cease acting as motors and become generators. This process, known as dynamic braking, is a form of electric braking that is used to reduce wear on the mechanical brake system components of a vehicle. In the case where the vehicle is a locomotive, dynamic braking reduces brake wear on the locomotive and also all of the rail cars of the train. Typically, a resistor is used to dissipate the electric power as heat produced by the electric motor during dynamic braking.
As a result, hybrid propulsion systems have been developed to recover some of the energy that is typically wasted as heat during dynamic braking. The recovery of this wasted energy is known as regenerative braking. Numerous configurations for hybrid propulsion systems for vehicles are known in the art. Generally, such propulsion systems utilize two different energy sources: a heat engine and a traction battery or other energy storage unit. The heat engine may include any engine that burns a fuel to produce mechanical work, such as an internal combustion engine, a turbine engine, a diesel engine, etc. The energy storage unit may include an electrically re-chargeable battery, an ultracapacitor, or a flywheel having a high power density. Hybrid systems are advantageous due to their ability to increase the fuel efficiency of the propulsion system and to reduce air pollution.
Heavy duty vehicles, such as transit buses, trucks, locomotives and off-highway vehicles generally utilize a series hybrid propulsion system, wherein the final drive to a vehicle axle comprises an electrical drive system. Conventionally, a series hybrid propulsion system typically includes an on-board energy source, such as a heat engine, coupled to an alternator that converts the mechanical output of the heat engine into an alternating current (AC). A rectifier is generally used to convert the AC output of the alternator into a direct current (DC). A portion of the DC output of the rectifier is used to charge an energy storage unit such as a traction battery, and a remaining portion is utilized to drive one or more electrical motors, such as a DC motor or an AC motor. Power output of the electrical motor(s) is transmitted to one or more vehicle axles via an electrical drive system.
During acceleration of the vehicle, or when the vehicle is climbing steep grades, the energy storage unit or traction battery is operating in a state of discharge, to augment electrical power output of the heat engine-alternator and thus provide high power levels for a period of time that depends on the rating of the energy storage unit. During braking, the energy storage unit or traction battery is operating in a state of re-charge to regeneratively capture a portion of the energy typically wasted during braking. The charge in the traction battery therefore needs to be optimally maintained to adequately provide for both modes: power discharge during acceleration and re-charge during regenerative braking. As described above, the charge in the traction battery in such systems is maintained by the on-board energy source.
In known series hybrid systems, the on-board energy source and associated controls are typically operated in a mode to control a state of charge in the energy storage unit or traction battery. One method of on-board energy source control is to operate the on-board energy source to maintain the traction battery's state of charge within a given range. In this approach, when a computed state of charge falls below a given set point, the on-board energy source is started and continues charging until the state of charge reaches an upper control limit. At this point, the on-board energy system control reduces the output power from the on-board energy source until recharging of the energy storage unit is stopped. One disadvantage with the above approach is that, in case the computed state of charge is in error, the on-board energy source may not properly charge the energy storage unit, leading either to an undercharge or an overcharge of the battery. These are both situations that will prematurely shorten the life of the energy storage system (traction battery) and may also cause a reduction in fuel economy.
There is, hence, a need to provide an improved control for the on-board power source, which is important to achieve high cycle life of the energy storage unit for economic viability.